Electrodes are important components for biosensor chips, in which they are typically used for transducing biological and/or chemical signals into measurable electrical signals, e.g. voltages or currents. For such biosensor applications, an electrode is preferred that exhibits a good sensitivity, selectivity, and stability for reliably detecting a biological and/or chemical signal over a specific period of time. State-of-the-art biosensors may rely on integrated complementary metal-oxide semiconductor (CMOS) circuitry for local signal conditioning, wireless communication, cell or particle manipulation and/or other optical, electrical and/or mechanical modalities. Therefore, a biosensor electrode is also preferably compatible with back-end-of-line CMOS processing.
Biosensor chips with integrated electrodes may find application in drug discovery pharmacology, neural interface systems, cell-based biosensors and electrophysiology research tools such as multi-electrode arrays. More specifically, implantable neural probes may require dense arrays of small electrodes, e.g. having an area in the range from about 25 μm2 to 144 μm2, which also have low electrode-tissue interface impedance in order to achieve reliable measurement of small neuronal action potentials at a high spatial resolution.
Similar to conventional capacitors, the electrode-tissue interface impedance is largely electrostatic in nature due to the formation of a space-charge region, e.g. a double-layer capacitance (DLC), at the solid-liquid interface. Hence, the impedance strongly depends on the total accessible external and internal surface area of the electrode, in which the external surface area is defined by the morphology and grain size of the electrode material and the internal surface area is a function of the porosity of the material. The total accessible area also depends on the hydrophilicity of the material, and hence also on the chemical composition of the material surface.
In addition to the electrostatic double-layer capacitance, some electrode materials can exhibit a so-called pseudo-capacitance (PC), in which the capacitive behaviour is not caused by static charge-separation, but by charge-transfer processes at the electrode surface such as electrosorption, e.g. H-atom adsorption on Pt, redox reactions, e.g. fast redox reactions accompanied by (de)protonation at the surface of transition metal oxides comprising multiple oxidation states, and ion intercalation, e.g. Li+-ion intercalation that lead to charge build-up at the electrode-electrolyte interface. Double-layer capacitance and pseudo-capacitance add up to a common capacitance value of an electrode, and both increase with increasing accessible surface area. Unlike the DLC, the PC can also be enhanced by chemically modifying, e.g. functionalizing, the electrode material in order to facilitate charge-transfer processes. For many biosensor applications, it may be desirable to increase the overall capacitance, and hence the accessible surface area, while minimizing the geometric area in order to improve sensitivity and spatial resolution.
Many electrode materials are known in the art which may be suitable for use in biosensors, for example thin-film materials such as Au, Pt, Ir, IrOx, and TiN, conducting polymers such as poly-(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole, and carbon-based materials such as carbon nanotubes and nanofibres.
Unfortunately, except for Ti and TiN, the above-mentioned materials may be incompatible with the strict contamination rules in standard back-end-of-line (BEOL) CMOS processing, e.g. excluding the use of Au, expensive, e.g. Pt and Ir, and/or difficult to integrate in standard CMOS processing, e.g. conducting polymers and carbon materials. TiN is a standard CMOS material which has for example been used, amongst others, as surface area-enhancing diffusion barrier and anti-reflective coating. It is also relatively cheap to fabricate and has a proven track record as electrode material in cardiac pacemakers due to its good biocompatibility, chemical and mechanical stability, and corrosion resistance. Furthermore, TiN is an excellent moisture barrier and its microcolumnar structure provides a high surface roughness and hence a large accessible surface area. These properties make it a very interesting candidate for integration in biosensor chips.
Commercial chips, such as the multi-electrode arrays distributed by Multichannel Systems, employ TiN as electrode material. For such chips, impedances of less than 1 MOhm may be achieved for 79 μm2 electrodes, e.g. using 900 nm thick TiN deposited using plasma-enhanced chemical vapour deposition (PECVD). Generally, increasing the TiN thickness also increases the surface roughness due to the formation of large columnar grains, and therefore also enhances the accessible surface area.
While using a TiN layer having a large thickness is a viable path towards increasing the accessible surface area and hence capacitance of electrodes, its fabrication is not standard in CMOS BEOL-processing, where a thickness of tens to a few hundreds of nm are common Depositing and patterning thick TiN elements may therefore imply a disadvantageously extensive process development and time-consuming hardware maintenance. Consequently, cost-effective approaches towards improving the overall TiN capacitance while minimizing the layer thickness may be preferred.
DE 4324185 discloses a stimulating electrode, for example for a pacemaker, comprising titanium and a porous layer of titanium nitride. The active surface area is increased, according to this disclosure, by introducing microstructures in the surface of the functional electrode member before coating with the porous material.